Process for selective phosphating of a composite metal construction

ABSTRACT

The present disclosure relates to a method of chemical pretreatment and selective phosphation of a composite metal construction comprising at least a portion made of aluminum and at least a portion made of zinc and optionally a further portion made of iron, which includes
         (I) treating the composite metal construction with an aqueous zinc phosphation composition that results in the formation of a surface-covering crystalline zinc phosphate layer and then—with an intervening water rinse operation—   (II) applying an aqueous acidic passivation composition,       

     The present disclosure also relates to a corresponding zinc phosphation composition, to a concentrate for production thereof, to a corresponding composite metal construction and to a method of using thereof.

The present invention relates to the anticorrosive treatment of composite metal constructions comprising metallic surfaces of aluminum, zinc and optionally iron in a multistage method. The method of the invention enables the selective zinc phosphation of the zinc and iron surfaces of the composite metal construction without deposition of significant amounts of zinc phosphate on the aluminum surfaces. In this way, the aluminum surface is available, in a subsequent method step, for passivation with, for example, conventional acidic and optionally silane-containing passivation compositions that generate a homogeneous, thin passive layer that provides protection from corrosion.

In the method of the invention, firstly the formation of phosphate crystal clusters on the aluminum surfaces and secondly the formation of specks on zinc surfaces are prevented. In particular, however, cryolite precipitation on the surface of the composite metal constructions, especially on the aluminum parts thereof, and preferably the poisoning of the phosphation bath by titanium, zirconium and hafnium, are reduced.

Accordingly, the present invention also relates to a zinc phosphation composition comprising water-soluble inorganic compounds of boron in an amount sufficient to suppress the formation of specks, but not exceeding values for which the zinc phosphation loses its selectivity for the zinc and iron surfaces of the composite metal construction.

In the field of automotive manufacture that is of particular relevance to the present invention, various metallic materials are increasingly being used and joined in composite structures. In bodywork construction, a wide variety of different steels are still used predominantly owing to their specific material properties, but there is also increasing use of lightweight metals such as aluminum that are particularly important for a considerable reduction in weight of the overall bodywork. In order to take account of this development, the aim is to develop new concepts for bodywork protection or to further develop existing methods and compositions for anticorrosive treatment of the untreated bodywork.

There is therefore a need for improved methods of pre-treatment of complex components, for example automobile bodywork, comprising not only parts made of aluminum but those made of steel and possibly galvanized steel. As a result of the overall pretreatment, a conversion layer or a passivation layer is to be generated on all the metal surfaces that occur, which is suitable as anticorrosive paint substrate, especially prior to cathodic electrocoating.

German published specification DE 19735314 proposes a two-stage method in which there is firstly a selective phosphation of the steel surfaces and galvanized steel surfaces of a chassis likewise having aluminum surfaces, followed by a treatment of the chassis with a passivation solution for anticorrosive treatment of the aluminum parts of the chassis. According to the teaching disclosed therein, the selective phosphation is achieved by lowering the pickling action of the phosphation solution. For this purpose, DE 19735314 teaches phosphation solutions having a free fluoride content of less than 100 ppm, where the source of the free fluoride is formed exclusively by water-soluble complex fluorides, especially of hexafluorosilicates in a concentration of 1 to 6 g/l.

The prior art discloses other two-stage pretreatment methods that likewise follow the concept of depositing a crystalline phosphate layer on the steel surfaces and any galvanized and alloy-galvanized steel surfaces in the first step and passivating the aluminum surfaces in a further subsequent step. These methods are disclosed in documents WO 1999/012661 and WO 2002/066702. In principle, the methods disclosed therein are performed in such a way that, in a first step, the steel surfaces or galvanized steel surfaces are phosphated selectively, and the phosphation is also maintained in the subsequent passivation in a second method step, while no phosphate crystals are formed on the aluminum surfaces. The selective phosphation of the steel surfaces and galvanized steel surfaces is possible by a temperature-dependent restriction of the proportion of free fluoride ions in the phosphation solutions, the free acid contents of which are set within a range from 0 to 2.5 points.

International application WO 2008/055726 discloses an at least one-stage method of selective phosphation of steel surfaces and galvanized steel surfaces of a composite construction comprising aluminum parts. This published specification teaches phosphation solutions comprising water-soluble inorganic compounds of the elements zirconium and titanium, the presence of which successfully prevents the phosphation of the aluminum surfaces.

European application EP 2588646 discloses a method of selective phosphation of steel and galvanized steel in metallic components that also have aluminum surfaces. By means of this method, moreover, the formation of phosphate crystal clusters on the aluminum surfaces and of specks on the galvanized steel surfaces are prevented. For this purpose, silicon is added to the zinc phosphation composition in the form of water-soluble inorganic compounds.

Proceeding from this prior art, the aim is to further develop the selective phosphation of steel and galvanized steel in the anticorrosive treatment of metallic components in hybrid composition, i.e. composite metal constructions that have aluminum surfaces, to the effect that an improvement in process economics during the phosphation is achieved by specific control of the bath parameters that control selectivity. With regard to the quality of the anticorrosive treatment of composite metal constructions, this includes not only the avoidance of the formation of phosphate crystal clusters on the aluminum surfaces and of specks on the galvanized steel surfaces, but especially also the reduction of cryolite precipitation on the surface of the metallic components, especially on the aluminum surfaces, and preferably the poisoning of the phosphation bath by titanium, zirconium and/or hafnium.

“Phosphate crystal clusters” are understood by the person skilled in the art to mean the individualized and locally limited deposition of phosphate crystals on metal surfaces (here: aluminum surfaces). Such “crystal clusters” are enclosed by a subsequent paint primer and constitute in-homogeneities in the coating that can both distort the uniform visual impression of the painted surfaces and cause spot damage to paintwork.

“Speck formation” is understood by the person skilled in the art to mean the phenomenon in phosphation of the local deposition of amorphous white zinc phosphate in an otherwise crystalline phosphate layer on the treated zinc surfaces or on the treated galvanized or alloy-galvanized steel surfaces. Speck formation is caused by a locally elevated pickling rate of the substrate. Such point defects in phosphation may be a starting point for the corrosive loss of adhesion of subsequently applied organic paint systems, and so the formation of specks should be largely avoided in practice.

Cryolite precipitation (deposition of solid Na₃AlF₆ and/or K₃AlF₆) on the surface of composite metal constructions, especially on the aluminum parts, results in a sandpaper-like nature of this surface, which is undesirable since it continues down to the surface of the ready-painted constructions. The result is paint faults that can be eliminated only at great cost and inconvenience, for example by sanding the regions in question. This is of course associated with corresponding labor and higher costs.

Cryolite precipitation proceeds by the following formula:

3Na⁺(aq)+Al³⁺(aq)+6F⁻(aq)→Na₃AlF₆(s)↓ or

The sodium ions in the phosphation composition originally from additions such as, in particular, additives and/or accelerators. Additives that should be particularly mentioned here are sodium fluoride, sodium nitrate, sodium phosphate and sodium hydroxide, and an accelerator that should be particularly mentioned is sodium nitrite. The same applies to potassium ions. These additions are added to the bath according to the situation and the existing bath parameters. The aluminum ions, by contrast, were removed from the aluminum parts of the composite metal constructions by pickling in the acidic medium of the phosphation bath composition.

A particular minimum amount of free fluoride (fluoride ions) in the phosphation bath is necessary to assure adequate deposition kinetics for the zinc phosphate layer on the surfaces of iron and zinc of the composite metal construction, since the simultaneous treatment of the aluminum surfaces of the composite metal construction especially causes aluminum ions to get into the zinc phosphation composition (see above), and these in turn, in uncomplexed form, disrupt or even prevent the formation of a uniform, homogeneous zinc phosphate layer on the iron and zinc parts of the composite construction. The free fluoride here may also come from fluoro complexes from which it is released in equilibrium reactions.

But the higher the free fluoride content for a given sodium/potassium and aluminum content the more significant the resulting cryolite precipitation at the surface of the composite metal constructions.

The cause of the poisoning of the phosphate bath by titanium, zirconium and/or hafnium may lie, in particular, in encrusted conveying systems for the composite metal constructions or in pre-passivated aluminum parts of the constructions.

The crusts (e.g. phosphate crusts or unbaked paint layers) on the conveying systems comprise titanium, zirconium and/or hafnium. The latter come from the passivation (cf. step (II)) with titanium-, zirconium- and/or hafnium-comprising water-soluble compounds that follows the phosphation (cf. step (I)) of the composite metal constructions, and they are absorbed by the crusts owing to their porosity. Since the conveying systems run in a cycle, the crusts that have been in contact with titanium, zirconium and/or hafnium in the passivation bath are exposed again to the acidic phosphation composition in the phosphation bath. Titanium, zirconium and/or hafnium are partly redissolved therein.

The aluminum portions of composite metal constructions are often pre-passivated, meaning that they have already been provided with a passivation layer comprising titanium, zirconium and/or hafnium that has already been applied to the aluminum portions by the suppliers thereof.

In both cases, there is complexation and hence leaching of titanium, zirconium and/or hafnium out of the crusts or the pre-passivation in the fluoride-containing phosphation bath. Consequently, titanium, zirconium and/or hafnium accumulate in the phosphation bath. This can result in deposition of black layers on the aluminum portions of the composite metal constructions, which can be wiped off and comprise amorphous zinc phosphate. Said layers can later lead to faults in the paint and also reduce protection from corrosion.

This above-described object is achieved in accordance with the invention by a method of chemical pretreatment and selective phosphation of a composite metal construction comprising at least a portion made of aluminum and at least a portion made of zinc and optionally a further portion made of iron, which comprises

-   -   (I) in a first step the treatment of the composite metal         construction with an aqueous zinc phosphation composition that         results in the formation of a surface-covering crystalline zinc         phosphate layer having a coat weight in the range from 0.5 to 5         g/m² on the parts made of iron and zinc, but does not produce a         zinc phosphate layer having a coat weight of at least 0.5 g/m²         on the aluminum parts, and then with an intervening water rinse         operation     -   (II) in a second step the application of an aqueous acidic         passivation composition having a pH in the range from 2.0 to 5.5         to the composite metal construction, where the acidic         passivation composition removes not more than 50% of the         crystalline zinc phosphate on the parts made of zinc and iron,         but forms a passive layer on the aluminum parts that does not         constitute a surface-covering crystalline phosphate layer,     -   where the zinc phosphation composition in step (I) comprises a         content of sodium and/or potassium ions and of aluminum ions,     -   where the zinc phosphation composition in step (I) has a         temperature in the range from 20 to 65° C. and comprises a free         fluoride content of at least 5 mg/l but not greater than 200         mg/l,     -   where at least 0.04 g/l but not more than 3.2 g/l of boron in         the form of water-soluble inorganic compounds, calculated as         BF₄, is present in the zinc phosphation composition, and     -   where the points number of free acid with added KCl in the zinc         phosphation composition is at least 0.6 points.

Definitions

A “composite metal construction” in the context of the present invention is a metallic component in hybrid composition, especially a chassis. The composite metal construction may, as well as surfaces of zinc, surfaces of aluminum and optionally surfaces of iron, also have surfaces of other metals and/or surfaces of nonmetals, especially of plastics.

According to the invention, the material “aluminum” is also understood to mean alloys thereof. At the same time, the material comprises zinc, according to the invention, and also comprises zinc alloys, for example zinc-magnesium alloys, and also galvanized steel and alloy-galvanized steel, while the mention of iron also includes iron alloys, especially steel. Alloys of the aforementioned materials have an extraneous atom content of less than 50% by weight.

In the present context, an “aqueous composition” is understood to mean a solution or dispersion comprising water as solvent or dispersion medium to a predominant extent, i.e. to an extent of more than 50% by weight, preferably to an extent of more than 70% by weight, more preferably to an extent of more than 90% by weight (based on the entirety of the solvents and dispersion media present). The aqueous composition may thus also comprise dispersed constituents as well as dissolved constituents. However, it is preferably a solution, i.e. a composition that comprises only small amounts of dispersed constituents, preferably no dispersed constituents at all, but rather comprises solely or almost solely dissolved constituents.

A “passive layer” in the context of the present invention is a passivating, i.e. anticorrosive, inorganic or mixed inorganic-organic thin-film coating that does not have a continuous crystalline phosphate layer. Such passive layers may consist, for example, of oxidic compounds of titanium, zirconium and/or hafnium.

According to the invention, “water-soluble compounds” means either one water-soluble compound or two or more water-soluble compounds.

The requirement that no zinc phosphate layer may form on the aluminum portions in the treatment step (I) should be understood such that no continuous and sealed crystalline layer is formed there. This condition is fulfilled at least when the area-based mass of zinc phosphate deposited on the aluminum portions is less than 0.5 g/m². Aluminum portions are understood in the context of the present invention to mean sheets and components made of aluminum and/or alloys of aluminum.

The formation of a continuous and crystalline zinc phosphate layer on the steel, galvanized steel and/or alloy-galvanized steel surfaces, by contrast, is absolutely necessary and characteristic of the method of the invention. For this purpose, zinc phosphate layers having an area-based coat weight of preferably at least 1.0 g/m², more preferably of at least 2.0 g/m², but preferably not more than 4.0 g/m², are deposited on these surfaces of the composite metal construction in step (I) of the method according to the invention.

For all surfaces of the composite metal construction, the zinc phosphate coat weight is determined with the aid of gravimetric difference weighing on test sheets of the individual metallic materials of the respective composite metal construction. This is done by contacting steel sheets, directly after a step (I), with an aqueous solution of 9% by weight of EDTA, 8.7% by weight of NaOH and 0.4% by weight of triethanolamine at a temperature of 70° C. for 5 minutes and in this way freeing them of the zinc phosphate layer. Analogously for the determination of the zinc phosphate coat weight on galvanized or alloy-galvanized steel sheets, a corresponding test sheet, immediately after a step (I), is contacted with an aqueous solution of 20 g/l of (NH₄)₂Cr₂O₇ and 27.5% by weight of NH₃ at a temperature of 25° C. for 4 minutes and in this way freed of the zinc phosphate layer.

Aluminum sheets, by contrast, immediately after a step (I), are contacted with an aqueous 65% by weight HNO₃ solution at a temperature of 25° C. for 15 minutes and correspondingly freed of zinc phosphate components. The difference in the weight of the dry metal sheets after this respective treatment from the weight of the same dry untreated metal sheet immediately prior to step (I) corresponds to the coat weight of zinc phosphate according to this invention.

The requirement of the invention that, in step (II), not more than 50% of the crystalline zinc phosphate layer on the steel and galvanized steel and/or alloy-galvanized steel surfaces is dissolved can likewise be appreciated using test sheets of the individual metallic materials of the respective composite metal construction. For this purpose, the test sheets of steel or galvanized or alloy-galvanized steel that had been phosphated in step (II) of the process of the invention, after a step of rinsing with deionized water, are blown dry with compressed air and then weighed.

The same test sheet is then contacted in step (II) of the process of the invention with the acidic passivation composition, then rinsed with deionized water, blown dry with compressed air and then weighed again. The zinc phosphation of the same test sheet is then removed completely with an aqueous solution of 9% by weight of EDTA, 8.7% by weight of NaOH and 0.4% by weight of triethanolamine or with an aqueous solution of 20 g/l of (NH₄)₂Cr₂O₇ and 27.5% by weight of NH₃, as described above, and the dry test sheet is weighed once more. The percentage loss of phosphate layer in step (II) of the process of the invention is then determined from the weighing differences of the test sheet.

The free acid with added KCl (FA, KCl) of the zinc phosphation composition in points in step (I) of the process of the invention is determined as follows (see: W. Rausch “Die Phosphatierung von Metallen” [The Phosphation of Metals], Eugen G. Leuze Verlag, 3rd edition, 2005, chapter 8.1, p. 333-334):

10 mL of the phosphation composition is pipetted into a suitable vessel, for example a 300 mL Erlenmeyer flask. If the phosphation composition comprises complex fluorides, an additional 6-8 g of potassium chloride is added to the sample. Titration then takes place, using a pH meter and an electrode, with 0.1 M NaOH to a pH of 4.0. The quantity of 0.1 M NaOH consumed in this titration, in ml per 10 ml of the phosphation composition, gives the free acid value with added KCl (FA, KCl) in points.

The zinc phosphation composition in step (I) preferably has a temperature in the range from 30 to 60° C., more preferably from 35 to 55° C.

The concentration of free fluoride in the zinc phosphation composition is determined in the process of the invention by means of a potentiometric method. This is done by taking a sample volume of the zinc phosphation composition and determining the activity of the free fluoride ions with any desired commercial fluoride-selective potentiometric combination electrode after calibration of the combination electrode by means of fluoride-containing standard solutions without pH buffering. Both the calibration of the combination electrode and the measurement of free fluoride are undertaken at a temperature of 20° C.

The exceedance of a limiting concentration of free fluoride dependent on the free acid, but in any case the exceedance of a concentration of 200 mg/l, causes the deposition of an area-covering crystalline zinc phosphate layer across the aluminum surfaces. However, such layer formation is undesirable owing to the substrate-specific coating properties of a zinc phosphation and is therefore not in accordance with the invention. However, a particular minimum amount of free fluoride is necessary to assure adequate deposition kinetics for the zinc phosphate layer on the surfaces of iron and zinc of the composite metal construction, since the simultaneous treatment of the aluminum surfaces of the composite metal construction especially causes aluminum cations to get into the zinc phosphation composition, and these in turn, in uncomplexed form, inhibit zinc phosphation.

The zinc phosphation composition therefore has a free fluoride content of at least 5 mg/l but not greater than 200 mg/l, preferably at least 10 mg/l but not greater than 200 mg/l, further preferably at least 20 mg/l but not greater than 175 mg/l, further preferably at least 20 mg/l but not greater than 150 mg/l, further preferably at least 20 mg/l but not greater than 135 mg/l, further preferably at least 20 mg/l but not greater than 120 mg/l, especially preferably at least 50 mg/l but not greater than 120 mg/l and very especially preferably at least 70 mg/l but not greater than 120 mg/l.

The total fluoride content (F_(tot.)) of the zinc phosphation composition is preferably in the range from 0.01 to 0.3 mol/I, more preferably from 0.02 to 0.2 mol/I.

The total fluoride content (F_(tot.)) is the content of fluoride (F⁻) in total and is composed of complex-bound fluoride and simple fluoride. Complex-bound fluoride is fluoride bound to Si and B, but also to Al and in further complexes. Simple fluoride, by contrast, is the fluoride not bound in complexes and is composed of the free fluoride determinable by means of a combination electrode, i.e. the unbound fluoride, and the fluoride bound as HF.

The higher the content of sodium and/or potassium ions, aluminum ions and free fluoride in the zinc phosphation composition in step (I) of the process of the invention, the more significant the precipitation of cryolite on the surface of the composite metal constructions, especially on the aluminum portions.

If the content of sodium and/or potassium ions (calculated as sodium) is in the range from 1 to 4 g/l, especially from 2 to 3 g/l, and the free fluoride content is in the range from 50 to 150 mg/l, especially from 70 to 120 mg/l, it is possible in the presence of aluminum ions to particularly effectively reduce the precipitation of cryolite by the presence of boron in the form of water-soluble inorganic compounds, especially in the form of fluoroborates such as HBF₄.

The zinc phosphation composition preferably has a concentration of boron in the form of water-soluble inorganic compounds calculated as BF₄ in the range from 0.08 to 2.5 g/l, further preferably from 0.08 to 2 g/l, further preferably from 0.25 to 2 g/l, especially preferably from 0.5 to 1.5 g/l and very especially preferably from 0.8 to 1.2 g/l.

The inventive addition of water-soluble inorganic compounds comprising boron additionally results in the suppression of speck formation on the zinc surfaces. The upper limit of 3.2 g/l calculated as BF₄ results firstly from the economic viability of the process and secondly from the fact that the process is made more difficult to control by such high concentrations of water-soluble inorganic compounds comprising boron since the formation of phosphate crystal clusters on the aluminum surfaces can be suppressed only to a minor degree via an increase in the free acid content.

The crystal clusters, on completion of paint application, result in point elevations that have to be sanded for visually homogeneous painting of the composite metal construction, for example an automobile chassis, as desired by the customer.

It has been found that, surprisingly, for effective suppression of the formation of a crystalline zinc phosphate layer and of zinc phosphate crystal clusters on the aluminum surfaces, the boron concentration in the form of water-soluble inorganic compounds calculated as BF₄ is a critical parameter crucial to the success of the process of the invention. If a concentration of 2.0 g/l is exceeded, there is already formation at least of individual zinc phosphate crystal clusters on the aluminum surfaces.

With further exceedance of this concentration, the aluminum surfaces in the process of the invention are coated with a crystalline zinc phosphate layer over the whole area. Both scenarios should be avoided for successful anticorrosive pretreatment. Therefore, in step (I) of the process of the invention, zinc phosphation compositions in which the concentration of boron in the form of water-soluble inorganic compounds does not exceed a value of 3.2 g/l, more preferably a value of 2.0 g/l each calculated as BF₄ are used.

In each case, however, the inventive proportion of boron in the form of water-soluble inorganic compounds is sufficient for prevention of speck formation on the zinc portions treated in accordance with the invention. Water-soluble inorganic compounds comprising boron that are preferred in the method of the invention are fluoroborates, more preferably HBF₄, (NH₄)BF₄, LiBF₄ and/or NaBF₄. The water-soluble fluoroborates are additionally suitable as a source of free fluoride and therefore serve to complex trivalent aluminum cations introduced into the bath solution, such that phosphation on the surfaces of steel and galvanized and/or alloy-galvanized steel is still assured.

In the case of the use of fluoroborates in phosphation solutions in step (I) of the process of the invention, it should of course always be ensured that the concentration of boron in the form of water-soluble inorganic compounds calculated as BF₄ does not exceed the value of 3.2 g/l according to claim 1 of the present invention.

It is possible that the zinc phosphation composition in step (I) includes, as well as water-soluble inorganic compounds comprising boron, also a certain content of water-soluble inorganic compounds comprising silicon. The latter may have been introduced into the phosphation composition for example, from preceding process steps or in the case of a high silicon content of the aluminum portions also originate from the composite metal constructions.

However, a content of inorganic compounds comprising silicon is firstly undesirable in the context of the present invention since this increases cryolite precipitation on the surface of the composite metal constructions, especially on the aluminum portions thereof, which can be explained as follows:

In the case of acidic pickling attack on the surface of the composite metal construction, protons are consumed in that they are reduced to hydrogen. This results in a rise in the pH at the surface. It has now been found in the present context that, surprisingly, fluorosilicates such as hexafluorosilicate, with rising pH, release fluoride ions (free fluoride) to a significantly greater degree than the corresponding fluoroborates such as tetrafluoroborate. Consequently, there is also a much more significant rise in the content of fluoride ions at the surface in the case of the fluorosilicates, which ultimately causes a corresponding increase in cryolite precipitation given an appropriate content of sodium/potassium and aluminum ions (cf. law of mass action).

A content of inorganic compounds comprising silicon is also secondly undesirable since it has now been found that, surprisingly, a zinc phosphation bath composition having a free fluoride content that includes silicon in the form of water-soluble inorganic compounds is capable of much more significant leaching of titanium, zirconium and/or hafnium out of the encrusted conveying systems for the composite metal constructions and/or the pre-passivated aluminum portions of the constructions, and hence can lead to significantly greater poisoning of the phosphation bath by titanium, zirconium and/or hafnium than one including boron in the form of water-soluble inorganic compounds.

This can again be explained in that, as already described further up above, as a result of the shift in pH at the surface of the composite metal construction, fluoride is released from fluoride complexes and can then complex titanium, zirconium and hafnium. As a result, these remain soluble and can accumulate in the bath. By comparison with fluorosilicates such as hexafluorosilicate, fluoroborates such as tetrafluoroborate release fluoride ions less readily, which means that less free fluoride is available for the complexation of titanium, zirconium and hafnium. Moreover, in the case of tetrafluoroborates, the concentration of free fluoride can be better controlled.

A content of inorganic compounds comprising silicon is thirdly undesirable since sparingly soluble precipitates form in the presence of potassium ions especially in the case of fluorosilicates such as H₂SiF₆. The formation of such precipitates is just as undesirable as the precipitation of cryolite. Potassium ions are advantageous compared to sodium ions as cations in salts for adjustment of the bath parameters, for example in phosphates, hydroxides and fluorides, since K₃AlF₆ is of better solubility than Na₃AlF₆ and hence less cryolite is precipitated. Although ammonium ions that may be present in the zinc phosphation composition also have this advantage (good solubility of (NH₄)₃AlF₆), they are however more environmentally harmful in the wastewater than potassium ions. The formation of sparingly soluble, potassium-containing precipitates has not been found for corresponding boron-comprising compounds especially in the case of fluoroborates such as HBF₄.

The content of inorganic compounds comprising silicon (calculated as SiF₆) is therefore preferably below 25 mg/l, further preferably below 15 mg/l, more preferably below 5 mg/l and most preferably below 1 mg/l.

The zinc phosphation composition in step (I) preferably has a points number of free acid with added KCl in the range from 0.6 to 3.0 points, preferably from 0.8 to 3.0 points, more preferably from 1.0 to 3.0 points and most preferably from 1.0 to 2.5 points. Observance of the preferred ranges for free acid firstly ensures adequate deposition kinetics of the phosphate layer on the selected metal surfaces and secondly prevents unnecessary pickling removal of metal ions, which in turn entails intensive monitoring or workup of the phosphation bath for avoidance of precipitation of sludges or for disposal thereof in continuous operation of the method of the invention.

In addition, total acid (TA) in the phosphation composition in step (I) of the method of the invention should be in the range from 10 to 50, further preferably from 15 to 40 and especially preferably from 20 to 35 points.

Fischer total acid (FTA), by contrast, should be in the range from 10 to 30, further preferably from 12 to 25 and more preferably from 15 to 20 points, while the A value should be in the range from 0.04 to 0.20, further preferably from 0.05 to 0.15 and especially preferably from 0.06 to 0.12.

It has been found that, surprisingly, both cryolite precipitation at the surface of the composite metal constructions and the poisoning of the phosphation bath by titanium, zirconium and/or hafnium can be reduced particularly effectively when, for the zinc phosphation composition in step (I), the dimensionless quotient

(A value×T)/(F_(tot)×[B]×1000)

is in the range from 0.13 to 22.5, preferably from 0.2 to 15 and more preferably from 0.4 to 10, where “T” represents the temperature/° C., “F_(tot.)” represents the total fluoride content/(mol/l) and “[B]” represents the concentration of the element boron/(mol/l).

Free acid (diluted) (FA (dil.)), which is required exclusively for determination of Fischer total acid (FTA), Fischer total acid (FTA), total acid (TA) and the A value are determined as follows:

Free Acid (Diluted) (FA (dil.)):

(See W. Rausch “Die Phosphatierung von Metallen”, Eugen G. Leuze Verlag, 3rd edition, 2005, section 8.1, pp. 333-334)

For determination of free acid (diluted), 10 ml of the phosphation composition is pipetted into a suitable vessel, such as a 300 ml Erlenmeyer flask. 150 ml of fully demineralized water is then added. Using a pH meter and an electrode, titration takes place with 0.1 M NaOH to a pH of 4.7. The quantity of 0.1 M NaOH consumed in this titration, in ml per 10 ml of the diluted phosphation composition, gives the value of free acid (diluted) (FA (dil.)) in points.

Fischer Total Acid (FTA):

(See W. Rausch “Die Phosphatierung von Metallen”, Eugen G. Leuze Verlag, 3rd edition, 2005, section 8.2, pp. 334-336)

Following determination of free acid (diluted), the diluted phosphation composition, following addition of potassium oxalate solution, is titrated, using a pH meter and an electrode, with 0.1 M NaOH to a pH of 8.9. The consumption of 0.1 M NaOH in this procedure, in ml per 10 ml of the diluted phosphation composition, gives Fischer total acid (FTA) in points.

Total Acid (TA):

(See W. Rausch “Die Phosphatierung von Metallen”, Eugen G. Leuze Verlag, 3rd edition, 2005, section 8.3, pp. 336-338)

Total acid (TA) is the sum of the divalent cations present and also free and bonded phosphoric acids (the latter being phosphates). It is determined by the consumption of 0.1 M NaOH, using a pH meter and an electrode. For this purpose, 10 ml of the phosphation composition is pipetted into a suitable vessel, such as a 300 ml Erlenmeyer flask, and diluted with 25 ml of fully demineralized water. This is followed by titration with 0.1 M NaOH to a pH of 9. The consumption during this procedure, in ml per 10 ml of the diluted phosphation composition, corresponds to the points number of the total acid (TA).

Acid Value (a Value):

(See W. Rausch “Die Phosphatierung von Metallen”, Eugen G. Leuze Verlag, 3rd edition, 2005, section 8.4, p. 338)

What is called the acid value (A value) is the ratio of free acid with added KCl to Fischer total acid, i.e. (FA, KCl):FTA, and is found by division of the value for free acid with added KCl (FA, KCl) by the Fischer total acid value (FTA).

The phosphation composition in step (I) preferably has a pH in the range from 2.5 to 3.5.

For an optimal phosphation outcome on metallic components having not only aluminum surfaces but also surfaces of steel and galvanized and/or alloy-galvanized steel, preference is given to zinc phosphation compositions in step (I) of the process of the invention that comprise a total of not more than 20 ppm, preferably not more than 15 ppm, further preferably not more than 10 ppm, more preferably not more than 5 ppm and most preferably a total of not more than 1 ppm of water-soluble compounds of zirconium, titanium and/or hafnium, based on the elements zirconium, titanium and/or hafnium, and especially preferably do not comprise any water-soluble compounds of zirconium, titanium and/or hafnium.

Consequently, the addition of water-soluble compounds of zirconium, titanium and/or hafnium in particular to the phosphation compositions in step (I) of the process of the invention is dispensed with entirely.

It is known from WO 2008/055726 that the presence of water-soluble compounds of these elements in a phosphation stage is likewise capable of effectively suppressing the formation of crystalline phosphate layers on aluminum surfaces. However, it has been found that, in the presence of water-soluble compounds of zirconium, titanium and/or hafnium, especially in the case of application of the phosphation composition by a spraying method, a comparatively frequent outcome is an inhomogeneous amorphous zirconium-, titanium- and/or hafnium-based conversion coating on the aluminum portions, which leads to the occurrence of “mapping” in the case of subsequent organic painting.

“Mapping” is understood by the person skilled in the art of dip coating of metallic components to mean a spotty visual impression of the paint coating owing to an inhomogeneous paint layer thickness after the baking of the dip paint.

In order to keep the concentration of zirconium, titanium and/or hafnium low, it has been found to be advantageous to reduce the free fluoride concentration or the fluoride concentration overall, the effect of which is that layer formation on the aluminum portions is suppressed and the process can be better controlled.

The cause of poisoning of the phosphate bath by titanium, zirconium and/or hafnium may as already described above lie, in particular, in encrusted conveying systems for the composite metal constructions or in pre-passivated aluminum parts of the constructions.

As set out further up, it is possible via the boron content of the zinc phosphation composition in the form of water-soluble inorganic compounds, especially via fluoroborates such as tetrafluoroborate in particular, to reduce the poisoning of the phosphation bath by titanium, zirconium and/or hafnium.

Moreover, in the case of high contents of free fluoride, leaching out of the crusts or the pre-passivation takes place to a more significant degree in the acidic medium. At the same time, the titanium and/or zirconium is stabilized by the high free fluoride content. The result is formation of the fluoro complexes of titanium and/or zirconium. Lowering the free fluoride content destabilizes the solubility of titanium and/or zirconium, which precipitates out and hence no longer constitute poisoning of the phosphation bath.

Lowering the free fluoride content does reduce the formation of a zinc phosphate layer on the aluminum portions. However, by virtue of subsequent passivation cf. step (II) this is not a problem. On the contrary, low zinc phosphation on the aluminum portions is beneficial in the corrosion and paint adhesion tests conducted.

The zinc phosphation composition therefore has a free fluoride content of at least 5 mg/l but not greater than 200 mg/l, preferably at least 10 mg/l but not greater than 200 mg/l, further preferably at least 20 mg/l but not greater than 175 mg/l, further preferably at least 20 mg/l but not greater than 150 mg/l, further preferably at least 20 mg/l but not greater than 135 mg/l, further preferably at least 20 mg/l but not greater than 120 mg/l, especially preferably at least 50 mg/l but not greater than 120 mg/l and very especially preferably at least 70 mg/l but not greater than 120 mg/l.

In a first preferred embodiment of the method of the invention, the conveying systems for the composite metal constructions have crusts that comprise titanium, zirconium and/or hafnium and preferably originate from the passivation of the composite metal constructions with titanium, zirconium- and/or hafnium-comprising water-soluble compounds in step (II) that follows the phosphation in step (I). The conveying systems preferably run in a cycle, such that the crusts that have come into contact with titanium, zirconium and/or hafnium in step (II) are exposed again to the acidic phosphation composition in step (I).

In a second preferred embodiment, the aluminum portions of the composite metal constructions have been pre-passivated, i.e. they have already been provided with a passivation layer comprising titanium, zirconium and/or hafnium.

In a third preferred embodiment, the conveying systems for the composite metal constructions have crusts that comprise titanium and/or zirconium and preferably originate from the passivation of the composite metal constructions with titanium-, zirconium- and/or hafnium-comprising water-soluble compounds in step (II) that follows the phosphation in step (I). The conveying systems preferably run in a cycle, such that the crusts generated in step (II) are exposed again to the acidic phosphation composition in step (I). In addition, the aluminum portions of the composite metal constructions have been pre-passivated, i.e. they have already been provided with a passivation layer comprising titanium, zirconium and/or hafnium.

The zinc phosphation composition in step (I) of the process of the invention preferably comprises at least 0.3 g/l, more preferably at least 0.8 g/l, but preferably not more than 3 g/l, more preferably not more than 2 g/l, of zinc ions. The content of phosphate ions in the phosphation solution is preferably in the range from 5 to 50 g/l, further preferably from 8 to 25 g/l and more preferably from 10 to 20 g/l in each case calculated as P₂O₅.

The zinc phosphation composition of the process of the invention may, as well as the aforementioned zinc ions and phosphate ions, additionally comprise at least one of the following accelerators:

-   -   0.07 to 0.25 g/l of nitrite (calculated as sodium nitrite),     -   about 0.1 to 2 g/l of nitroguanidine     -   8 to 51 mg/l of hydrogen peroxide.

Such accelerators are familiar in the art as components of phosphation baths and fulfill the function of “hydrogen scavengers” in that they directly oxidize hydrogen formed by the acidic attack on the metallic surface and in so doing are themselves reduced. The forming of a homogeneous crystalline zinc phosphate layer is significantly facilitated by the accelerator that prevents the formation of gaseous hydrogen at the metal surface.

The zinc phosphation composition of the process of the invention may, as well as the aforementioned zinc ions and phosphate ions, additionally comprise at least one of the following anions:

-   -   0 to 2 g/l of sulfate,     -   0.01 to 500 mg/l (in the case of nickel-free zinc phosphation         composition) or 0.01 to 20 g/l (in the case of nickel-containing         zinc phosphation composition) of nitrate.

The proportion of up to 5 g/l, preferably up to 3 g/l, of nitrate which is customary in nickel-containing phosphation facilitates the formation of a crystalline homogeneous and continuous phosphate layer on steel and on galvanized and alloy-galvanized steel surfaces.

In the case of nickel-free phosphation, however, the acceleration of the layer formation reaction by nitrate is critical. Such a nickel-free phosphation leads to lower coat weights and in particular to reduced incorporation of manganese into the crystal. Since the phosphate coating has a zero nickel content, however, too low a manganese content of the phosphate coating is at the expense of its alkali resistance.

The alkali resistance in turn plays a critical part during subsequent cathodic electrocoat deposition. In this process, electrolytic dissociation of water occurs at the substrate surface: hydroxide ions are formed. As a result, the pH at the substrate interface goes up. This makes agglomeration and deposition of the electrocoat possible in the first place; but the elevated pH can also damage the crystalline phosphate layer.

Experience has shown that corrosion protection and paint adhesion of the crystalline zinc phosphate layers produced with an aqueous phosphation composition of the invention are improved when one or more of the following cations is additionally present:

-   -   0.3 to 2 g/l, preferably 0.5 to 1.5 g/l, of manganese(II),     -   0.3 to 2 g/l, preferably 0.8 to 1.2 g/l, of nickel(II),     -   0.001 to 0.2 g/l, preferably 0.005 to 0.1 g/l, of iron(III).

Firstly, iron(III) cations result in improved corrosion protection on steel and on zinc and zinc alloys. Secondly, iron(III) cations lead to improved stability of the corresponding phosphation bath. The addition of iron(III) cations can additionally generate a more easily filterable sludge floc, such that cryolite can be more easily removed from the bath.

Aqueous compositions for conversion treatment that comprise not only zinc ions but also both manganese and nickel ions, as tricationic phosphation solutions, are known to the person skilled in the art in the field of phosphation and are also of good suitability in the context of the present invention.

As well as the aforementioned cations that are incorporated into the phosphate layer or at least have a positive effect on the crystal growth of the phosphate layer, the phosphation solutions in step (I) of the process of the invention, as well as sodium and/or potassium ions, optionally also comprise ammonium ions that pass into the phosphation solution for adjustment of the free acid content.

The water rinse operation between step (I) and step (II) is obligatory since the phosphate ions and metal cations present in the phosphation composition would otherwise be entrained into the passivation bath. This would lead to a change in the bath parameters that would be difficult to control and to precipitation of titanium, zirconium and/or hafnium as sparingly soluble phosphates.

The water rinse operation may be one water rinse or else a totality of at least two water rinses.

Each water rinse may be conducted with fully demineralized water or tap water. There may also be at least one surfactant present in each case.

After the workup of the rinse water enriched with the components of the phosphation bath in step (I), selective recycling of the components into the phosphation bath may be undertaken.

The composite metal construction treated and rinsed in step (I) and is contacted in step (II) with the acidic passivation composition by dipping or by spray application of the solution.

In step (II) of the method, by virtue of the contacting of the composite metal construction with the acidic passivation composition, in accordance with the invention, a passivation layer is formed on the aluminum surfaces, with dissolution of the zinc phosphate layer on the steel surfaces or galvanized and/or alloy-galvanized steel surfaces during the contacting with the passivation composition to an extent of not more than 50%, preferably to an extent of not more than 20%, more preferably to an extent of not more than 10%. In the context of the present invention, passive layers of aluminum are considered to be passivating inorganic or mixed inorganic-organic thin-film coatings that do not form continuous crystalline phosphate layers.

While the pH of the acidic passivation composition in the range from 2.0 to 5.5, preferably from 2.5 to 5.5, more preferably from 3.0 to 5.5 and most preferably from 3.5 to 5.5 essentially ensures that not more than 50% of the zinc phosphate layer on the steel surfaces or galvanized and/or alloy-galvanized steel surfaces is dissolved, the corresponding passive layers on the aluminum surfaces of the composite metal construction are typically generated by chromium-free acidic passivation compositions.

The passivation composition comprises at least one water-soluble compound selected from the water-soluble compounds of the metal ions of Mo, Cu, Ag, Au, Ti, Zr, Hf, Pd, Sn, Sb, Li, V and Ce. The water-soluble compounds are respectively present in a concentration in the following preferred, particularly preferred and very particularly preferred ranges (each calculated as the corresponding metal):

Mo 1 to 1000 mg/l 10 to 500 mg/l 20 to 225 mg/l Cu 1 to 1000 mg/l 100 to 500 mg/l 150 to 225 mg/l Ag 1 to 500 mg/l 5 to 300 mg/l 20 to 150 mg/l Au 1 to 500 mg/l 10 to 300 mg/l 20 to 200 mg/l Ti 20 to 500 mg/l 50 to 300 mg/l 50 to 150 mg/l Zr 20 to 500 mg/l 50 to 300 mg/l 50 to 150 mg/l Hf 20 to 500 mg/l 50 to 300 mg/l 50 to 150 mg/l Pd 1 to 200 mg/l 5 to 100 mg/l 15 to 60 mg/l Sn 1 to 500 mg/l 2 to 200 mg/l 3 to 100 mg/l Sb 1 to 500 mg/l 2 to 200 mg/l 3 to 100 mg/l Li 1 to 200 mg/l 5 to 180 mg/l 10 to 150 mg/l V 20 to 500 mg/l 50 to 300 mg/l 50 to 150 mg/l Ce 20 to 500 mg/l 50 to 300 mg/l 50 to 150 mg/l

The metal ions present in the water-soluble compounds are deposited either in the form of a salt which comprises the corresponding metal cation (e.g. molybdenum or tin) preferably in at least two oxidation stages more particularly in the form of an oxyhydroxide, a hydroxide, a spinel or a defect spinel or in elemental form on the surface that is to be treated (e.g. copper, silver, gold or palladium).

In a first preferred embodiment, the passivation composition comprises molybdenum ions and optionally further metal ions each in the form of water-soluble compounds. They are added to the passivation composition preferably in the form of molybdate, further preferably of ammonium heptamolybdate and especially preferably of ammonium heptamolybdate×7H₂O. The molybdenum ions may also be added in the form of sodium molybdate.

Molybdenum ions may alternatively be added to the passivation composition, for example, in the form of at least one salt comprising molybdenum cations, such as molybdenum chloride, and then oxidized to molybdate by a suitable oxidizing agent, examples being the accelerators described earlier on above. In such a case, the passivation composition itself comprises a corresponding oxidizing agent.

Further preferably, the passivation composition comprises molybdenum ions in combination with copper ions, tin ions or zirconium ions.

With particular preference it comprises molybdenum ions in combination with zirconium ions and also, optionally, comprises a polymer or copolymer selected more particularly from the group consisting of the polymer classes of the polyamines, polyethyleneamines, polyanilines, polyimines, polyethyleneimines, polythiophenes and polypyrroles and also mixtures thereof and copolymers thereof and polyacrylic acid.

The content of molybdenum ions here is preferably in the range from 20 to 225 mg/l, more preferably from 50 to 225 mg/l and very preferably from 100 to 225 mg/l (calculated as Mo), and the content of zirconium ions is preferably in the range from 50 to 300 mg/l, more preferably from 50 to 150 mg/l (calculated as Zr).

In a second preferred embodiment, the passivation composition comprises copper ions, especially copper(II) ions, and optionally other metal ions each in the form of water-soluble compounds. Preferably, the passivation composition in that case comprises a content of copper ions in the range from 100 to 500 mg/l, more preferably from 150 to 225 mg/l.

In a particularly preferred embodiment, the passivation composition comprises Ti, Zr and/or Hf in complexed form, preferably in a concentration in the range from 20 to 500 mg/l, further preferably from 50 to 300 mg/l and more preferably from 50 to 150 mg/l (calculated as Ti, Zr and/or Hf), and optionally further metal ions in the form of water-soluble compounds. The complexes in question are preferably fluoro complexes, more preferably hexafluoro complexes. Moreover, the passivation composition comprises preferably 10 to 500 mg/l, further preferably 15 to 100 mg/l and especially preferably 15 to 50 mg/l of free fluoride.

In a first very particularly preferred embodiment, the passivation composition comprises, as well as Ti, Zr and/or Hf in complex form, especially as fluoro complexes, and additionally at least one water-soluble compound selected from the water-soluble compounds of the metal ions of molybdenum, copper, especially copper(II), silver, gold, palladium, tin and antimony, and also lithium, preferably of molybdenum and copper, especially copper(II). The water-soluble compounds are respectively present in a concentration in the following preferred ranges (each calculated as the corresponding metal):

Mo 20 to 225 mg/l Cu 150 to 225 mg/l Ag 20 to 150 mg/l Au 20 to 200 mg/l Pd 15 to 60 mg/l Sn 3 to 100 mg/l Sb 3 to 100 mg/l Li 1 to 200 mg/l

In a second very particularly preferred embodiment, the passivation composition comprises, as well as Ti, Zr and/or Hf in complex form, especially as fluoro complexes, additionally at least one organosilane and/or at least one hydrolysis product thereof, i.e. an organosilanol, and/or at least one condensation product thereof, i.e. an organosiloxane/polyorganosiloxane, preferably in a concentration in the range from 5 to 200 mg/l, further preferably from 10 to 100 mg/l and especially preferably from 20 to 80 mg/l (calculated as Si).

The at least one organosilane preferably has at least one amino group. Particular preference is given to one that can be hydrolyzed to an aminopropylsilanol and/or to 2-aminoethyl-3-aminopropylsilanol and/or is a bis(trimethoxysilylpropyl)amine.

In a third very particularly preferred embodiment, the passivation composition comprises, as well as Ti, Zr and/or Hf in complex form, especially as fluoro complexes, additionally at least one water-soluble compound selected from the water-soluble compounds of the metal ions of molybdenum, copper, especially copper(II), silver, gold, palladium, tin and antimony, and also lithium, preferably of molybdenum and copper, especially copper(II), and at least one organosilane and/or at least one hydrolysis product thereof, i.e. an organosilanol, and/or at least one condensation product thereof, i.e. an organosiloxane/polyorganosiloxane, preferably in a concentration in the range from 5 to 200 mg/l, further preferably from 10 to 100 mg/l and especially preferably from 20 to 80 mg/l (calculated as Si). The water-soluble compounds are respectively present in a concentration in the following preferred ranges (each calculated as the corresponding metal):

Mo 20 to 225 mg/l Cu 1 to 30 mg/l Ag 20 to 150 mg/l Au 20 to 200 mg/l Pd 15 to 60 mg/l Sn 3 to 100 mg/l Sb 3 to 100 mg/l Li 1 to 200 mg/l

The process of the invention for anticorrosive treatment of composite metal constructions assembled from metallic materials that also have aluminum surfaces at least to some degree, after cleaning and activation of the metallic surfaces, is effected firstly by contacting the surfaces with the zinc phosphation composition of step (I), for example by a spraying or dipping method, at temperatures in the range of 20-65° C. and for a period of time matched to the mode of application. Experience has shown that speck formation on the galvanized and/or alloy-galvanized steel surfaces in conventional dip phosphation methods is particularly marked, and so the phosphation in step (I) of the process of the invention is suitable especially also for those phosphation plants that work by the principle of the dipping method, since speck formation in the method of the invention is suppressed.

It is advantageous first to clean and especially to degrease the composite metal construction prior to the treatment with the zinc phosphation composition in step (I). For this purpose, it is especially possible to use an acidic, neutral, alkaline or strongly alkaline cleaning composition, but optionally also additionally an acidic or neutral pickling composition.

An alkaline or strongly alkaline cleaning composition has been found here to be especially advantageous.

The aqueous cleaning composition may, as well as at least one surfactant, optionally also comprise a detergent builder and/or other additions, for example complexing agents. It is also possible to use an activating detergent.

After the cleaning/pickling, the composite metal construction is then advantageously at least rinsed with water, in which case the water may optionally have been admixed as well with a water-dissolved additive such as a nitrite or surfactant, for example.

Prior to the treatment of the composite metal construction with the zinc phosphation composition in step (I), it is advantageous to treat the composite metal construction with an activation composition. The purpose of the activating composition is to deposit a multitude of ultrafine phosphate particles as seed crystals on the surface of the portions made of zinc and iron. These crystals help to form an in particular crystalline phosphate layer having an extremely high number of densely disposed, fine phosphate crystals, or a largely continuous phosphate layer on the portions made of zinc and iron, in the subsequent method step, in contact with the phosphation composition—preferably without rinsing in-between.

Activating compositions contemplated in this case include, in particular, acidic or alkaline compositions based on titanium phosphate or zinc phosphate.

It may, however, also be advantageous to add activating agents, especially titanium phosphate or zinc phosphate, in the cleaning composition—in other words, to carry out cleaning and activation in one step.

In a further step that follows step (II), the composite metal construction may be provided with a primer coat, preferably with an organic dip coat, especially an electrocoat preferably without prior oven drying of the component treated in accordance with the invention. After the primer coat, it is possible to apply a top coat, which may be a powder coat or a wet coat.

The present invention additionally relates to a zinc phosphation composition for selective phosphation of steel, galvanized steel and/or alloy-galvanized steel surfaces in a metallic composite construction comprising a portion made of aluminum, wherein the zinc phosphation composition has a points number of free acid with added KCl of at least 0.6 points and a pH preferably in the range from 2.5 to 3.5, and

-   -   (a) 5 to 50 g/l of phosphate ions calculated as P₂O₅,     -   (b) 0.3 to 3 g/l of zinc ions,     -   (c) at least 5 mg/l but not more than 200 mg/l of free fluoride,         and     -   (d) at least 0.04 g/l but not more than 3.2 g/l of boron in the         form of water-soluble inorganic compounds, calculated as BF₄,

where the zinc phosphation composition additionally comprises a content of sodium and/or potassium ions and of aluminum ions.

Preferably, the free fluoride content is in the range from 10 to 200 mg/l, further preferably from 20 to 175 mg/l, further preferably from 20 to 150 mg/l, further preferably from 20 to 135 mg/l, further preferably from 20 to 120 mg/l, especially preferably from 50 to 120 mg/l and very especially preferably from 70 to 120 mg/l.

It has been found that, surprisingly, both cryolite precipitation at the surface of the composite metal constructions and the poisoning of the phosphation bath by titanium, zirconium and/or hafnium can be reduced particularly effectively when, for the zinc phosphation composition, the dimensionless quotient

(A value×T)/(F_(tot)×[B]×1000)

is in the range from 0.13 to 22.5, preferably from 0.2 to 15 and more preferably from 0.4 to 10, where “T” represents the temperature/° C., “F_(tot.)” represents the total fluoride content/(mol/l) and “[B]” represents the concentration of the element boron/(mol/l).

In a preferred variant, the zinc phosphation composition of the invention comprises a total of not more than 20 ppm, preferably not more than 15 ppm, further preferably not more than 10 ppm, more preferably not more than 5 ppm and most preferably not more than 1 ppm in total of water-soluble compounds of zirconium, titanium and/or hafnium, based on the elements zirconium, titanium and/or hafnium, and especially no water-soluble compounds of zirconium, titanium and/or hafnium.

Further advantageous configurations of the zinc phosphation composition of the invention have already been elucidated above in the method of the invention.

The present invention also relates to a concentrate from which the zinc phosphation composition of the invention can be obtained by diluting with a suitable solvent and/or dispersion medium, preferably with water, and optionally adjusting the pH.

The dilution factor here is preferably between 2 and 100, preferably between 5 and 50.

The present invention finally relates to a corrosion-protected composite metal construction which comprises at least a portion of aluminum and at least a portion of zinc and optionally a further portion of iron, and is obtainable by the method of the invention.

The composite metal constructions protected against corrosion by the method of the invention correspondingly find use in the automotive supplier industry, in bodywork construction in automobile manufacture, in agricultural machine construction, in shipbuilding, in the construction sector or for the production of white goods.

The present invention is to be illustrated by the nonlimiting working examples and comparative examples that follow.

EXAMPLES

Aqueous zinc phosphation solutions were made up, each of which had a points number of free acid with added KCl of 1.1 points, a Fischer total acid of 19.5 points, an A value of 0.056 and a pH in the range from 2.5 to 3.5. Said phosphation solutions were nickel-free and each comprised 15 g/l of phosphate ions calculated as P₂O₅, 0.8 g/l of zinc ions, 1.0 g/l of manganese ions, different amounts of free fluoride (see tab. 1), 1.0 g/l of boron in the form of and calculated as BF₄, and 34 mg/l of the hydrogen peroxide accelerator. The phosphation solutions were brought to a temperature of 45° C.

Cleaned test sheets of aluminum (AA 6016) were each sprayed uniformly with one phosphation solution, rinsed and dipped at room temperature into an acidic aqueous passivation composition comprising 150 mg/l of H₂ZrF₆ calculated as Zr for 30 s. Without prior oven drying, the sheets were provided with an electrocoat and finally with a wet coat (standard automobile paint structure).

The test sheets were subjected to a 144-hour CASS test to DIN EN ISO 9227, 2017-07 and to a 1008-hour filiform test to DIN EN 3665, 1997-08. The corrosion results are compiled in the table below (F_(free)=free fluoride content).

TABLE 1 Filiform Maximum thread CASS Average in mm length in mm F_(free) in mg/l Max. (vertical score) (vertical score) mm²/cm 50 1.0 1.0 5.7 23.1 100 2.0 1.8 7.7 38.3 150 2.8 2.6 13.2 51.2 200 3.5 3.1 11.0 72.5

As shown by tab. 1, lowering of the free fluoride content in the phosphation solution leads to a distinct reduction in corrosion on aluminum both in the CASS test and in the filiform test.

The applicant took samples over a period of 2.2 years from a customers operating zinc phosphation bath with varying free fluoride content and determined the content of free fluoride (F_(free)) and of zirconium (Zr). For this purpose, a fluoride-selective potentiometric combination electrode was used, or an ICP (inductively coupled plasma) analysis was performed.

As apparent from tab. 2, a decrease in the free fluoride content albeit with a time delay in some cases (cf. 10 to 26 months) was accompanied by a decrease in the zirconium content in the bath. Conversely, an increase in the free fluoride content albeit with a time delay in some cases (cf. 4.5 to 14.5 months) was accompanied by an increase in the zirconium content in the bath. A low free fluoride content thus led to lower bath poisoning by zirconium.

TABLE 2 Time in months F_(free) in mg/l Zr in mg/l 0 160 7.5 1 100 7.0 3 68 5.0 4.5 55 4.0 6 220 6.5 8.5 250 7.0 9 230 8.0 10 240 8.0 12 100 10.0 14.5 100 9.0 17 100 8.0 19 59 3.0 21 51 3.5 24 46 3.0 26 40 4.0

Aqueous zinc phosphation solutions (ZPS No. 1-8) were made up, each of which had a points number of free acid with added KCl of 1.5 points. Said phosphation solutions were nickel-free and each comprised 15 g/l of phosphate ions calculated as P₂O₅, 0.8 g/l of zinc ions, 1.0 g/l of manganese ions, 100 mg/l of free fluoride and different amounts of sodium and potassium ions, and also either tetrafluoroborate or hexafluorosilicate. The phosphation solutions were brought to a temperature of 45° C. and stirred in a beaker by means of a stir bar at moderate speed. By means of ICP analysis, the dissolved concentrations of aluminum, silicon and boron were determined at the start (0 h) and after 24 h. When the concentration of dissolved aluminum fell, this was attributable to precipitation in the form of cryolite. If, by contrast, there was a decrease in the concentration of dissolved silicone, this was based on precipitation in the form of K₂SiF₆.

As apparent from tab. 3, a content of 5.0 g/l of sodium ions led to a decrease in the concentration of dissolved aluminum, i.e. to precipitation of cryolite (ZPSs 2 and 6). Cryolite precipitation in the presence of tetrafluoroborate (ZPS 2) was somewhat less marked than in the case of hexafluorosilicate (ZPS 6). The same is true of a content of 2.5 g/l of sodium ions and 2.5 g/l potassium ions (ZPS 4 vs. ZPS 8). In the case of a content of 2.5 g/l of sodium ions (ZPSs 1 and 5) or of 5.0 g/l potassium ions (ZPSs 3 and 7), by contrast, there was no cryolite precipitation.

In the case of hexafluorosilicate, in the presence of potassium ions, there was a decrease in the concentration of dissolved silicon, i.e. precipitation of K₂SiF₆ (ZPSs 7 and 8). It was not possible to detect the formation of a corresponding precipitate for tetrafluoroborate. There was correspondingly no decrease here in the concentration of dissolved boron (ZPSs 3 and 4).

TABLE 3 ZPS Na K BF₄ SiF₆ Al (mg/l) Si (mg/l) B (mg/l) No. (g/l) (g/l) (g/l) (g/l) 0 h 24 h 0 h 24 h 0 h 24 h 1 2.5 0 1.0 0 47 47 0 0 145 145 2 5.0 0 1.0 0 46 23 0 0 150 150 3 0 5.0 1.0 0 46 46 0 0 148 148 4 2.5 2.5 1.0 0 46 25 0 0 150 150 5 2.5 0 0 1.5 46 46 300 300 0 0 6 5.0 0 0 1.5 47 22 290 290 0 0 7 0 5.0 0 1.5 47 46 200 9 0 0 8 2.5 2.5 0 1.5 46 22 300 80 0 0 

1. A method of chemical pretreatment and selective phosphation of a composite metal construction comprising at least a portion made of aluminum and at least a portion made of zinc and optionally a further portion made of iron, wherein the method comprises: (I) treating the composite metal construction with an aqueous zinc phosphation composition that results in a formation of a surface-covering crystalline zinc phosphate layer having a coat weight in a range from 0.5 to 5 g/m² on the parts made of iron and zinc, but does not produce a zinc phosphate layer having a coat weight of at least 0.5 g/m² on the aluminum parts, and then—with an intervening water rinse operation— (II) applying an aqueous acidic passivation composition having a pH in a range from 2.0 to 5.5 to the composite metal construction, where the aqueous acidic passivation composition removes not more than 50% of the crystalline zinc phosphate on the parts made of zinc and iron, but forms a passive layer on the aluminum parts that does not constitute a surface-covering crystalline phosphate layer, wherein the aqueous zinc phosphation composition in step (I) comprises a content of sodium and/or potassium ions and of aluminum ions, wherein the aqueous zinc phosphation composition in step (I) has a temperature in a range from 20 to 65° C. and comprises a free fluoride content of at least 5 mg/l but not greater than 200 mg/1, where wherein at least 0.04 g/l but not more than 3.2 g/l of boron in a form of water-soluble inorganic compounds, calculated as BF₄, is present in the aqueous zinc phosphation composition, and wherein a points number of free acid with added KCl in the aqueous zinc phosphation composition is at least 0.6 points.
 2. The method according to claim 1, wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 10 to 200 mg/l.
 3. The method according to claim 1, wherein the aqueous zinc phosphation composition in step (I) has a concentration of boron in the form of water-soluble inorganic compounds calculated as BF₄ in a range from 0.08 to 2.5 g/l.
 4. The method according to claim 1, wherein the content of sodium and/or potassium ions (calculated as sodium) in the aqueous zinc phosphation composition in step (I) is in a range from 1 to 4 g/l, and the free fluoride content is in the range from 50 to 150 mg/l.
 5. The method according to claim 1, wherein a content of inorganic compounds comprising silicon (calculated as SiF₆) in the aqueous zinc phosphation composition in step (I) is below 25 mg/l.
 6. The method according to claim 1, wherein the aqueous zinc phosphation composition in step (I) has a points number of free acid with added KCl in a range from 0.6 to 3.0 points.
 7. The method according to claim 1, wherein for the aqueous zinc phosphation composition in step (I) a dimensionless quotient (A value×T)/(F_(tot)×[B]×1000) is in a range from 0.13 to 22.5.
 8. The method according to claim 1, wherein the aqueous zinc phosphation composition in step (I) additionally includes one or more of the following cations: 0.3 to 2 g/l of manganese(II), 0.3 to 2 g/l of nickel(II), 0.001 to 0.2 g/l of iron(III).
 9. The method according to claim 1, wherein a pH of the aqueous acidic passivation composition in step (II) is in a range from 2.5 to 5.5.
 10. The method according to claim 1, wherein the aqueous acidic passivation composition in step (II) comprises Ti, Zr and/or Hf in complexed form and optionally further metal ions in a form of water-soluble compounds.
 11. The method according to claim 1, wherein the aqueous acidic passivation composition in step (II) additionally comprises at least one organosilane and/or at least one hydrolysis product thereof and/or at least one condensation product thereof.
 12. A zinc phosphation composition for selective phosphation of steel, galvanized steel and/or alloy-galvanized steel surfaces in a metallic composite construction comprising a portion made of aluminum according to claim 1, wherein the zinc phosphation composition has a points number of free acid with added KCl of at least 0.6 points, and (a) 5 to 50 g/l of phosphate ions calculated as P₂O₅, (b) 0.3 to 3 g/l of zinc ions, (c) at least 5 mg/l but not more than 200 mg/l of free fluoride, and (d) at least 0.04 g/l but not more than 3.2 g/l of boron in the form of water-soluble inorganic compounds, calculated as BF₄, where the zinc phosphation composition additionally comprises a content of sodium and/or potassium ions and of aluminum ions.
 13. A concentrate from which the zinc phosphation composition according to claim 12 can be obtained by diluting with a suitable solvent and/or dispersion medium and optionally adjusting the pH.
 14. A composite metal construction comprising at least a portion made of aluminum and at least a portion made of zinc and optionally a further portion made of iron, wherein it is obtainable by a method according to claim
 1. 15. A method of using the composite metal construction according to claim 14, the method comprising incorporating the composite metal construction into a component used in an automotive supplier industry, in bodywork construction in automobile manufacture, in agricultural machine construction, in shipbuilding, in a construction sector or for production of white goods.
 16. The method according to claim 2, wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 20 to 150 mg/l.
 17. The method according to claim 2, wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 20 to 120 mg/l.
 18. The method according to claim 2, wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 50 to 120 mg/l.
 19. The method according to claim 2, wherein the aqueous zinc phosphation composition in step (I) has a free fluoride content in a range from 70 to 120 mg/l.
 20. The method according to claim 3, wherein the aqueous zinc phosphation composition in step (I) has a concentration of boron in the form of water-soluble inorganic compounds calculated as BF₄ in a range from 0.08 to 2 g/l. 